Nickel base or cobalt base superalloys have been cast in investment molds in vacuum or air and then are moved to cool in air where exothermic material hot topping is applied to the mold pour cup to produce certain equiaxed grain cast gas turbine blades that are free of solidification shrinkage defects. For example, in casting such turbine blades, prior art workers have placed exothermic material, such as aluminum-containing powder material, on the molten superalloy reservoir remaining in the pour cup of the investment mold to keep molten after the mold is filled with molten superalloy and as soldification occurs in order to counter soldification shrinkage in the cast blade. This casting practice in air using such exothermic material is disadvantageous for several reasons that include, but are not limited to, occurrence of severe reactions (flash and burning) of the exothermic material upon contact with the molten superalloy in the mold pour cup as well as the need to safely remove the smoke and vapors from the containment area. Exposing a hot casting to air also promotes the formation of unwanted hafnium oxides as surface scale at last-to-solidify regions of the cast blade, such as the blade root when cast in the tip-down orientation. In addition, contamination of the superalloy material remaining in the pour cup from the reaction with the exothermic material occurs to such an extent that the contaminated pour cup material cannot be reused as revert (recycled) material in the casting of another part.
The use of exothermic material is described in U.S. Pat. No. 6,446,698 wherein a modified mold is used for casting molten metal or alloy. In particular, the mold is modified to have a destructible extension between the mold pour cup and a reservoir above the mold cavity and through which extension exothermic material is introduced and placed on the surface of the molten metal or alloy in the reservoir.
U.S. Pat. No. 3,841,384 describes a casting process sans exothermic material wherein an upper/lower split induction coil is used to heat a crucible placed on top of a mold to be cast. One of the coils is energized to first heat the crucible to melt a solid metal or alloy charge therein and then both coils are energized to impart superheat to the melt in the crucible and to preheat the mold for casting to receive molten metal or alloy from the crucible.
U.S. Pat. Nos. 5,592,984; 6,019,158; and 6,640,877 describe casting methods sans exothermic material for reducing shrinkage defects upon solidification of molten metal or alloy in a preheated mold by pressurizing the casting chamber or by placing a pressurizing cap on the mold after it is filled with molten metal. The entire mold is preheated prior to casting with no further mold heating.
U.S. Pat. No. 4,832,112 discloses the MX casting process sans exothermic material wherein a molten metal or alloy with controlled low superheat is cast into a mold and subjected to electromagnetic stirring to induce turbulence in the molten metal or alloy in the mold without substantial heating thereof.